1. Field of the Invention
The present invention relates to a method of annealing a semiconductor film with the use of laser light (hereinafter referred to as laser annealing) and to a laser apparatus for performing the laser annealing (an apparatus including a laser and an optical system for leading laser light output from the laser to a process object). The invention also relates to a semiconductor device fabricated by a manufacturing process that comprises the laser annealing step, and to the manufacturing process. The semiconductor device here includes an electro-optical device such as a liquid crystal display device and an EL display device, and an electronic device having the electro-optical device as one of its components.
2. Description of the Related Art
An advance has been made in recent years in development of thin film transistors (hereinafter referred to as TFTs), and TFTs using polycrystalline silicon films (polysilicon films) as crystalline semiconductor films are receiving the attention. In liquid crystal display devices (liquid crystal displays) and EL (electroluminescence) display devices (EL displays), in particular, such TFTs are used as elements for switching pixels and elements for forming driver circuits to control the pixels.
General means for obtaining a polysilicon film is a technique in which an amorphous silicon film is crystallized into a polysilicon film. A method in which an amorphous silicon film is crystallized with the use of laser light has lately become the one that is especially notable. In this specification, to crystallize an amorphous semiconductor film with laser light to obtain a crystalline semiconductor film is called laser crystallization.
The laser crystallization is capable of instantaneous heating of semiconductor film, and hence is an effective technique as measures for annealing a semiconductor film formed on a low heat resistant substrate such as a glass substrate or a plastic substrate. In addition, the laser annealing makes the throughput definitely higher as compared with conventional heating measures using an electric furnace (hereinafter referred to as furnace annealing).
There are various kinds of laser light, of which the general one to be used in laser crystallization is laser light generated and emitted from a pulse oscillation type excimer laser as a source (hereinafter referred to as excimer laser light). The excimer laser has advantages in that it is large in output and that it is capable of repetitive irradiation at a high frequency and, moreover, excimer laser light is advantageous in terms of its high absorption coefficient with respect to silicon films.
To generate excimer laser light, KrF (wavelength, 248 nm) or XeCl (wavelength, 308 nm) is used as an excitation gas. However, Kr (krypton) gas and Xe (xenon) gas are very expensive, causing a problem of increase in production cost when recharge of the gas is frequent.
In addition, every two or three years, excimer laser annealing requires replacement of attachments such as a laser tube for laser oscillation and a gas refinery for removing unnecessary compounds that are produced during the course of oscillation. Many of these attachments are also expensive, taking part in increasing the production cost.
As seen in the above, a laser apparatus using excimer laser light does possess high ability but also possess drawbacks in that maintenance thereof is very troublesome and that the running cost (which means the costs required for operating the apparatus) is high for a laser apparatus for mass production.